RFC 1615

Migrating from X.400(84) to X.400(88)

Network Working Group J. Houttuin
Request for Comments: 1615 RARE Secretariat
RARE Technical Report: 9 J. Craigie
Category: Informational Joint Network Team
May 1994 Migrating from X.400(84) to X.400(88)
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Scope
In the context of a European pilot for an X.400(88) messaging
service, this document compares such a service to its X.400(84)
predecessor. It is aimed at a technical audience with a knowledge of
electronic mail in general and X.400 protocols in particular.
Abstract
This document compares X.400(88) to X.400(84) and describes what
problems can be anticipated in the migration, especially considering
the migration from the existing X.400(84) infrastructure created by
the COSINE MHS project to an X.400(88) infrastructure. It not only
describes the technical complications, but also the effect the
transition will have on the end users, especially concerning
interworking between end users of the 84 and the 88 services.
Table of Contents
1. New Functionality 22. OSI Supporting Layers 33. General Extension Mechanism 54. Interworking 54.1. Mixed 84/88 Domains 54.2. Generation of OR-Name Extensions from X.400(84) 64.3. Distribution List Interworking with X.400(84) 84.4. P2 Interworking 105. Topology for Migration 116. Conclusion 127. Security Considerations 13
Appendix A - DL-expanded and Redirected Messages in X.400(84) 14
Appendix B - Bibliography 14
Appendix C - MHS Terminology 15

Appendix D - Abbreviations 16
Authors' Addresses 171. New Functionality
Apart from the greater maturity of the standard and the fact that it
makes proper use of the Presentation Layer, the principal features of
most use to the European R&D world in X.400(88) not contained in
X.400(84) are:
- A powerful mechanism for arbitrarily nested Distribution
Lists including the ability for DL owners to control access
to their lists and to control the destination of nondelivery
reports. The current endemic use of DLs in the research
community makes this a fundamental requirement.
- The Message Store (MS) and its associated protocol, P7. The
Message Store provides a server for remote User Agents (UAs)
on Workstations and PCs enabling messages to be held for
their recipient, solving the problems of non-continuous
availability and variability of network addresses of such
UAs. It provides powerful selection mechanisms allowing the
user to select messages from the store to be transferred to
the workstation/PC. This facility is not catered for
adequately by the P3 protocol of X.400(84) and provides a
major incentive for transition.
- Use of X.500 Directories. Support for use of Directory Names
in MHS will allow a transition from use of O/R Addresses to
Directory Names when X.500 Directories become widespread,
thus removing the need for users to know about MHS
topological addressing components.
- The provision of message Security services including
authentication, confidentiality, integrity and non-
repudiation as well as secure access between MHS components
may be important for a section of the research community.
- Redirection of messages, both by the recipient if
temporarily unable to receive them, and by the originator in
the event of failure to deliver to the intended recipient.
- Use of additional message body encodings such as ISO 8613
ODA (Office Document Architecture) reformattable documents or
proprietary word processor formats.

- Physical Delivery services that cater for the delivery of an
electronic message on a physical medium (such as paper)
through the normal postal delivery services to a recipient
who (presumably) does not use electronic mail.
- The use of different body parts. In addition to the
extensible externally defined body parts, the most common
types are predefined in the standard. In order to give end-
users a real advantage as compared to other technologies, the
following body-parts should be supported as a minimum:
- IA5
- Message
- G3FAX
- External
- General Text
- Others
The last bullet should be interpreted as follows: all UAs
should be configurable to use any type of externally defined
body part, but as a minimum General Text can be generated and
read.
- The use of extended character sets, both in O/R addresses
and in the General Text extended bodypart. As a minimum, the
character sets as described in the European profiles will be
supported. A management domain may choose as an internal
matter which character sets it wants to support in
generating, but all user agents must be able to copy (in
local address books and in replies) any O/R address, even if
it contains character sets it cannot interpret itself.
2. OSI Supporting Layers
The development of OSI Upper Layer Architecture since 1984 allows the
new MHS standards to sit on the complete OSI stack, unlike X.400(84).
A new definition of the Reliable Transfer Service (RTS), ISO 9066,
has a mode of operation, Normal-mode, which uses ACSE and the OSI
Presentation Layer. It also defines another mode compatible with
X.410(84) RTS that was intended only for interworking with X.400(84)
systems.
However, there are differences between the conformance requirements
of ISO MOTIS and CCITT X.400(88) for mandatory support for the
complete OSI stack. ISO specify use of Normal-mode RTS as a mandatory
requirement with X.410-mode RTS as an additional option, whereas
CCITT require X.410-mode and have Normal-mode optional. The ISO
standard identifies three MTA types to provide options in RTS modes:

- MTA Type A supports only Normal-mode RTS, and provides
interworking within a PRMD and with other PRMDs (conforming
to ISO 10021), and with ADMDs which have complete
implementations of X.400(88) or which conform to ISO 10021.
- MTA Type B adds to the functionality of MTA type A the
ability to interwork with ADMDs implementing only the minimal
requirements of X.400(88), by requiring support for X.410-
mode RTS in addition to Normal-mode.
- MTA Type C adds to the functionality of MTA type B the
ability to interwork with external X.400(84) Management
Domains (MDs, i.e., PRMDs and ADMDs), by requiring support for
downgrading (see 5.1) to the X.400(84) P1 protocol.
The interworking between ISO and CCITT conformant systems is
summarised in the following table:
CCITT
X.400(84) X.400(88)
minimal complete
implementation
ISO 10021/ MTA Type A N N Y
MOTIS MTA Type B N Y Y
MTA Type C Y Y Y
Table 1: Interworking ISO <-> CCITT systems
The RTS conformance difference would totally prevent interworking
between the two versions of the standard if implementations never
contained more than the minimum requirements for conformance, but in
practice many 88 implementations will be extensions of 84 systems,
and will thus support both modes of RTS. (At the moment we are aware
of only one product that doesn't support Normal mode.)
Both ISO and CCITT standards require P7 (and P3) to be supported
directly over the Remote Operations Service (ROS), ISO 9072, and use
Normal-mode presentation (and not X.410-mode). Both allow optionally
ROS over RTS (in case the Transport Service doesn't provide an
adequately reliable service), again using Normal-mode and not X.410-
mode.
CCITT made both Normal and X.410 mode mandatory in its X.400(92)
version, and it is expected that the 94 version will have the X.410
mode as an option only.

3. General Extension Mechanism
One of the major assets in ISO 10021/X.400(88) is the extension
mechanism. This is used to carry most of the extensions defined in
these standards, but its principal benefit will be in reducing the
trauma of transitions to future versions of the standards. Provided
that implementations of the 88 standards do not try to place
restrictions on the values that may be present, any future extension
will be relayed by these implementations when appropriate (i.e., when
the extension is not critical), thus providing a painless migration
to new versions of the standards.
4. Interworking
4.1. Mixed 84/88 Domains
ISO 10021-6/X.419(88) defines rules for interworking with X.400(84),
called downgrading. As X.400 specifies the interconnection of MDs,
these rules define the actions necessary by an X.400(88) MD to
communicate with an X.400(84) MD. The interworking rules thus apply
at domain boundaries. Although it would not be difficult to extend
these to rules to convert Internal Trace formats which might be
thought a sufficient addition to allow mixed X.400(84)/X.400(88)
domains, the problems involved in attempting to define mixed 84/88
domains are not quite that simple.
The principle problem is in precisely defining the standard that
would be used between MTAs within an X.400(84) domain, as X.400(84)
only defines the interconnection of MDs. In practice, MTA
implementations either use X.400(84) unmodified, or X.400(84) with
the addition of Internal Trace from the first MOTIS DIS (DIS 8883),
or support MOTIS as defined in DIS 8505, DIS 8883, and DIS 9065. The
second option is recommended in the NBS Implementors Agreements, and
the third option is in conformance with the CEN/CENELEC MHS
Functional Standard [1], which requires as a minimum tolerance of all
"original MOTIS" protocol extensions. An 84 MD must decide which of
these options it will adopt, and then require all its MTAs to adopt
(or at least be compatible with) this choice. No doubt this is one of
the reasons for the almost total absence currently of mixed- vendor
X.400(84) MDs in the European R&D MHS community. The fact that none
of these three options for communication between MTAs within a domain
have any status within the standardisation bodies accounts for the
absence from ISO 10021/X.400(88) of detailed rules for interworking
within mixed 84/88 domains.
Use of the first option, unmodified X.400(84), carries the danger of
undetectable routing loops occurring. Although these can only occur
if MTAs have inconsistent routing tables, the absence of standardised

methods of disseminating routing information makes this a possibility
which if it occurred might cause severe disruption before being
detected. The only addition to the interworking rules needed for this
case is the deletion of Internal Trace when downgrading a message.
Use of the second option, X.400(84) plus Internal Trace, allows the
detection and prevention of routing loops. Details of the mapping
between original-MOTIS Internal Trace and the Internal Trace of ISO
10021 can be found in Annex A. This should be applied not only when
downgrading from 88 to 84, but also in reverse when an 84 MPDU is
received by the 84/88 Interworking MTA. If the 84 domain properly
implements routing loop detection algorithms, then this will allow
completely consistent reception of messages by an 84 recipient even
after DL expansion or Redirection within that domain (but see also
section 5.3). Unfortunately, the first DIS MOTIS like X.400(84) left
far too much to inference, so not all implementors may have
understood that routing loop detection algorithms must only consider
that part of the trace after the last redirection flag in the trace
sequence.
Use of the third option, tolerance of all original-MOTIS extensions,
would in principle allow a still higher level of interworking between
the 84 and 88 systems. However, no implementations are known which do
more than relay the syntax of original-MOTIS extensions: there is no
capability to generate these protocol elements or ability to
correctly interpret their semantics.
The choice between the first two options for mixed domains can be
left to individual management domains. It has no impact on other
domains provided the topology recommended in section 5 is adopted.
4.2. Generation of OR-Name Extensions from X.400(84)
The interworking rules defined in DIS 10021-6/X.419 Annex B allow for
delivery of 88 messages to 84 recipients, but do not make any 88
extensions available to 84 originators. In general this is an
adequate strategy. Most 88 extensions provide optional services or
have sensible defaults. The exception to this is the OR-Name
extensions. These fall into three categories: the new CommonName
attribute; fifteen new attributes for addressing physical delivery
recipients; and alternative Teletex (T.61) encodings for all
attributes that were defined as Printable Strings. Without some
mechanism to generate these attributes, 84 originators are unable to
address 88 recipients with OR-Addresses containing these attributes.
Such a mechanism is defined in RARE Technical Report 3 ([2]), "X.400
1988 to 1984 downgrading".

Common-name appears likely to be a widely used attribute because it
remedies a serious deficiency in the X.400(84) OR-Name: it provides
an attribute suitable for naming Distribution Lists and roles, and
even individuals where the constraints of the 84 personal-name
structure are inappropriate or undesirable. As 84 originators will no
doubt wish to be able to address 88 DLs (and roles), [2] defines a
Domain Defined Attribute (DDA) to enable generation of common-name by
84 originators. This consists of a DDA with its type set to "common-
name" and its value containing the Printable String encoding to be
set into the 88 common-name attribute.
This requires that all European R&D MHS 88 MTAs capable of
interworking with 84 systems shall be able to map the value of
"common-name" DDA in OR-Names received from 84 systems to the 88
standard attribute extension component common-name, and vice versa.
X.400(84) originators will only be able to make use of this ability
to address 88 common-name recipients if their system is capable of
generating DDAs. Unfortunately, one of the many serious deficiencies
with the CEN/CENELEC and CEPT 84 MHS Functional Standards ([1] and
[3]), as originally published, is that this ability is not a
requirement for all conformant systems. Thus if existing European R&D
MHS X.400(84) users wish to be able to address a significant part of
the ISO 10021/X.400(84) world they must explicitly ensure that their
84 systems are capable of generating DDAs. However, this will be a
requirement in the revised versions of ENV 41201 and ENV 41202, which
are to be published soon. There is no alternative mechanism for
providing this functionality to 84 users. It is estimated that
currently 95% of all European R&D MHS users are able to generate
DDAs.
When messages are sent to both ISO 10021/X.400(88) and X.400(84)
recipients outside the European R&D MHS community, this
representation of common-name will not enable the external recipients
to communicate directly unless their 84/88 interworking MTA also
implements this mapping. However, use of this mapping within the
European R&D MHS community has not reduced external connectivity, and
provided RTR 3, RFC 1328 is universally implemented within this
community it will enhance connectivity within the community.
As for the new Physical Delivery address attributes in X.400(88), RTR
3 (RFC1328) takes the following approach. A DDA with type "X400-88"
is used, whose value is an std-or encoding of the address as defined
in RARE Technical Report 2 ([4]), "Mapping between X.400(1988)/ISO
10021 and RFC 822". This allows source routing through an appropriate
gateway. Where the generated address is longer than 128 characters,
up to three overflow DDAs are used: X400-C1; X400-C2; X400-C3. This
solution is general, and does not require co-operation, i.e., it can

be implemented in the gateways only.
Note that the two DDA solutions mentioned above have the undesirable
property that the P2 heading will still contain the DDA form, unless
content upgrading is also done. In order to shield the user from
cryptic DDAs, such content upgrading is in general recommended, also
for nested forwarded messages, even though the available standards
and profiles do not dictate this.
4.3. Distribution List Interworking with X.400(84)
Before all X.400(84) systems are upgraded to ISO 10021, the
interaction of Distribution Lists with X.400(84) merits special
attention as DLs are already widely used.
Nothing, apart perhaps from the inability to generate the DL's OR-
Address if the DL uses the common-name attribute, prevents an
X.400(84) originator from submitting a message to a DL.
X.400(84) users can also be members (i.e., recipients) of a DL.
However, if the X.400(84) systems involved correctly implement
routing loop detection, the X.400(84) recipient may not receive all
messages sent to the DL. X.400(84) routing loop detection involves a
recipient MD in scanning previous entries in a message's trace
sequence for an occurrence of its own domain, and if such an entry is
found the message is non-delivered. The new standards extend the
trace information to contain flags to indicate DL-expansion and
redirection, and re-define the routing loop detection algorithm to
only examine trace elements from the last occurrence of either of
these flags. Thus 88 systems allow a message to re-traverse an MD (or
be relayed again by an MTA) after either DL-expansion or redirection.
However, these flags cannot be included in X.400(84) trace, so are
deleted on downgrading. Therefore the 84 DL recipient will receive
all messages sent to the DL except those which had a common point in
the path to the DL expansion point with the path from the expansion
points to his UA. This common point is an MD in the case of a DL in
another MD or an MTA in the case of a DL in the same MD. Although
this is quite deterministic behaviour, the user is unlikely to
understand it and instead regard it as erratic or inconsistent
behaviour.
Another problem with X.400(84) DL members will be that delivery and
non-delivery reports will be sent back directly to the originator of
a message, rather than routed through the hierarchy of DL expansion
points where they could have been routed to the DL administrator
instead of (or as well as) the originator.

No general solution to this problem has yet been devised, despite
much thought from a number of experts. The nub of the problem is that
changing the downgrading rules to enable 84 recipients to receive all
such messages also allows the possibility of undetectable infinite DL
or redirection looping where there is an 84 transit domain.
A potential solution is to extend the DL expansion procedures to
explicitly identify X.400(84) recipients and to treat them specially,
at least by deleting all trace prior to the expansion point. This
solution is only dangerous if another DL reached through an 84
transit domain is inadvertently configured as an 84 recipient, when
infinite looping could occur. It does however impose the problems of
84 interworking into MHS components which need to know nothing even
of the existence of X.400(84). It also requires changes to the
Directory attribute mhs-dl-members to accommodate the indication that
identifies the recipient as an X.400(84) user, unless European R&D
MHS DLs are restricted to being implemented by local tables rather
than making use of the Directory.
A similar change would be required for Redirection. However, the
change for Redirection would have substantially more impact as it
would require European R&D MHS-specific MHS protocol extensions to
identify the redirected recipient as an X.400(84) user. If the
European R&D MHS adopts a reasonable quality of MHS(88) service, all
its MTAs would be capable of Redirection and all UAs would be capable
of requesting originator-specified-alternate-recipient and thus be
required to incorporate these non-standard additions. A special
European R&D MHS modification affecting all MTAs and UAs seems
impractical, too!
If the recommended European R&D MHS topology for MHS migration (See
chapter 5) is adopted there will never be an X.400(84) transit domain
(or MTA) between two ISO 10021 systems. This allows the deletion of
trace prior to the last DL expansion or redirection to be performed
as part of the downgrading, giving the X.400(84) user a consistent
service. This solution has the advantage of only requiring changes at
the convertors between X.400(84) and ISO 10021/X.400(88), where other
European R&D MHS specific extensions have also been identified. A
precise specification of this solution is given in Annex A.
Finally, problems might occur because some X.400(84) MTAs could
object to messages containing more than one recipient with the same
extension-id (called originally-requested-recipient-number in the new
standards), since this was not defined in X.400(84). Note that
X.400(84) only requires that all extension-id's be different at
submission time, so 84 software that does not except messages with
identical extension-id's for relaying or delivery must be considered
broken.

4.4. P2 Interworking
RTR 3, RFC 1328 also defines the downgrading rules for P2 (IPM)
interworking: The IPM service in X.400(1984) is usually provided by
content type 2. In many cases, it will be useful for a gateway to
downgrade P2 from content type 22 to 2. This will clearly need to be
made dependent on the destination, as it is quite possible to carry
content type 22 over P1(1984). The decision to make this downgrade
will be on the basis of gateway configuration.
When a gateway downgrades from 22 to 2, the following should be done:
1. Strip any 1988 specific headings (language indication, and
partial message indication).
2. Downgrade all O/R addresses, as described in Section 3.
3. If a directory name is present, there is no method to
preserve the semantics within a 1984 O/R Address. However, it
is possible to pass the information across, so that the
information in the Distinguished Name can be informally
displayed to the end user. This is done by appending a text
representation of the Distinguished Name to the Free Form
Name enclosed in round brackets. It is recommended that the
"User Friendly Name" syntax is used to represent the
Distinguished Name [5]. For example:
(Steve Hardcastle-Kille, Computer Science,
University College London, GB)
4. The issue of body part downgrade is discussed in Section 6.
Note that a message represented as content type 22 may have
originated from [6]. The downgrade for this type of message can be
improved. This is discussed in RTR 2, RFC 1327.
Note that the newer EWOS/ETSI recommendations specify further rules
for downgrading, which are not all completely compatible with the
rules in RTR 3, RFC 1328. This paper does not state which set of
rules is preferred for the European R&D MHS, it only states that a
choice will have to be made.
As the transition topology recommended for the European R&D MHS is to
never use 84 transit systems between 88 systems, it is possible to
improve on the P2 originator downgrading and resending scenario. The
absence of 84 transit systems means that the necessity for a P1
downgrade implies that the recipient is on an 84 system, and thus
that it is better to downgrade 88 P2 contents to 84 P2 rather than to

relay it in the knowledge that it will not be delivered.
5. Topology for Migration
Having decided that a transition from X.400(84) is appropriate, it is
necessary to consider the degree of planning and co- ordination
required to preserve interworking during the transition.
It is assumed as a fundamental tenet that interworking must be
preserved during the transition. This requires that one or more
system in the European R&D MHS community must act as a protocol
converter by implementing the rules for "Interworking with 1984
Systems" listed in Annex B of ISO 10021-6/X.419.
When downgrading from ISO 10021/X.400(88) to X.400(84) all extensions
giving functionality beyond X.400(84) are discarded, or if a critical
extension is present then downgrading fails and a non-delivery
results. Thus, although it is possible to construct topologies of
interconnected MTAs so that two 88 MTAs can only communicate by
relaying through one or more 84 MTA, to maximise the quality of
service which can be provided in the European R&D MHS community it is
proposed that it require that no two European R&D MHS 88 MTAs shall
need to communicate by relaying through a X.400(84) MTA. Furthermore,
if this is extended to require that no two European R&D MHS 88 MTAs
shall ever communicate by relaying through an X.400(84) MTA, then the
European R&D MHS can provide enhanced interworking functionality to
its X.400(84) users.
If mixed vintage 88 and 84 Management Domains (MDs) are created, the
routing loop detection rules, which specify that a message shall not
re-enter an MD it has previously traversed, require that downgrading
is performed within that mixed vintage MD. That MD therefore requires
at least one MTA capable of downgrading from 88 to 84. It is unlikely
that every MTA within an MD will be configured to act as an entry-
point to that MD from other MDs. However, the proposed European R&D
MHS migration topology requires that as soon as a domain has an 88
MTA it shall also have an 88 entry point - this may, of course, be
that same MTA.
Even for MDs operating all the same MHS vintage internally, providing
entry-points for both MHS vintages will give considerable advantage
in maximising the connectivity to other MDs. Initially, it will be
particularly important for 88 MDs to be able to communicate with 84
only MDs, but as 88 becomes more widespread eventually the 84 MDs
will become a minority for which the ability to support 88 will be
important to maintain connectivity. For most practical MDs providing
entry-points that implement options in the supporting layers will
also be important. Support for at least the following is recommended

at MD entry-points:
88-P1/Normal-mode RTS/CONS/X.25(84)
88-P1/Normal-mode RTS/RFC1006/TCP/IP
84-P1/X.25(80)
84-P1/RFC1006/TCP/IP
The above table omits layers where the choice is obvious (e.g.,
Transport class zero), or where no choice exists (e.g., RTS for 84-
P1).
The requirement for no intermediate 84 systems does require that the
European R&D MHS use direct PRMD to PRMD routing between 88 PRMDs at
least until such time as all ADMDs will relay the 88 MHS protocols.
Finally, in order to keep routing co-ordination overhead to a
minimum, an important requirement for the migration strategy is that
only one common set of routing procedures is used for both 84 and 88
systems in the European R&D MHS.
6. Conclusion
1. The transition from X.400(84) to ISO 10021/X.400(88) is
worthwhile for the European R&D MHS, to provide:
- P7 Message Store to support remote UAs.
- Distribution Lists.
- Support for Directory Names.
- Standardised external Body Part types.
- Redirection.
- Security.
- Future extensibility.
- Physical Delivery.
2. To minimise the number of transitions the European R&D MHS
target should be ISO 10021 rather than CCITT X.400(88) -
i.e., straight to use of the full OSI stack with Normal-mode
RTS.
3. To give a useful quality of service, the European R&D MHS
should not use minimal 88 MTAs which relay the syntax but
understand none of the semantics of extensions. In
particular, all European R&D MHS 88 MTAs should generate
reports containing extensions copied from the subject message
and route reports through the DL expansion hierarchy where
appropriate.

4. The European R&D MHS should carefully plan the transition so
that it is never necessary to relay through an 84 system to
provide connectivity between any two 88 systems.
5. The European R&D MHS should consider the additional
functionality that can be provided to X.400(84) users by
adopting an enhanced specification of the interworking rules
between X.400(84) and ISO 10021/X.400(88), and weigh this
against the cost of building and maintaining its own
convertors. The advantages to X.400(84) users are:
- Ability to generate 88 common-name attribute, likely to
be widely used for naming DLs.
- Consistent reception of DL-expanded and Redirected
messages.
- Ability to receive extended 88 P2 contents
automatically downgraded to 84 P2.
7. Security Considerations
Security issues are not discussed in this memo.

[10] CCITT Recommendations X.400 - X.430, "Data Communication
Networks: Message Handling Systems", CCITT Red Book, Vol. VIII -
Fasc. VIII.7, Malaga-Torremolinos, 1984.
[11] CCITT Recommendations X.400 - X.420 (ISO IS-10021), "Data
Communication Networks: Message Handling Systems", CCITT Blue
Book, Vol. VIII - Fasc. VIII.7, Melbourne, 1988.
Appendix C - MHS Terminology
Message Handling is performed by four types of functional entity:
User Agents (UAs) which enable the user to compose, send, receive,
read and otherwise process messages; Message Transfer Agents (MTAs)
which provide store-and-forward relaying services; Message Stores
(MSs) which act on behalf of UAs located remotely from their
associated MTA (e.g., UAs on PCs or workstations); and Access Units
(AUs) which interface MHS to other communications media (e.g., Telex,
Teletex, Fax, Postal Services). Each UA (and MS, and AU) is served by
a single MTA, which provides that user's interface into the Message
Transfer Service (MTS).
Collections of MTAs (and their associated UAs, MSs and AUs) which are
operated by or under the aegis of a single management authority are
termed a Management Domain (MD). Two types of MD are defined: an
ADMD, which provides a global public message relaying service
directly or indirectly to all other ADMDs; and a PRMD operated by a
private concern to serve its own users.
A Message is comprised of an envelope and its contents. Apart from
the MTS content-conversion service, the content is not examined or
modified by the MTS which uses the envelope alone to provide the
information required to convey the message to its destination.
The MTS is the store-and-forward message relay service provided by
the set of all MTAs. MTAs communicate with each other using the P1
Message Transfer protocol.